MECHANICAL AND METALLURGICAL ANALYSIS OF STRUCTURAL STEEL pot

NIST NCSTAR 1-3 Federal Building and Fire Safety Investigation of the World Trade Center Disaster Mechanical and Metallurgical Analysis of Structural Steel... NIST NCSTAR 1-3 Federal B

NIST NCSTAR 1-3

Federal Building and Fire Safety Investigation of the World Trade Center Disaster

Mechanical and Metallurgical

Analysis of Structural Steel

NIST NCSTAR 1-3

Federal Building and Fire Safety Investigation of the World Trade Center Disaster

Mechanical and Metallurgical

Analysis of Structural Steel

Materials Science and Engineering Laboratory

National Institute of Standards and Technology

September 2005

U.S Department of Commerce

Carlos M Gutierrez, Secretary

Technology Administration

Michelle O’Neill, Acting Under Secretary for Technology

Disclaimer No 1

Certain commercial entities, equipment, products, or materials are identified in this document in order to describe a procedure or concept adequately or to trace the history of the procedures and practices used Such identification is not intended to imply recommendation, endorsement, or implication that the entities, products, materials, or

equipment are necessarily the best available for the purpose Nor does such identification imply a finding of fault or negligence by the National Institute of Standards and Technology

designed or installed as required by a code provision, NIST has documentary or anecdotal evidence indicating

whether the requirement was met, or NIST has independently conducted tests or analyses indicating whether the requirement was met

Use in Legal Proceedings

No part of any report resulting from a NIST investigation into a structural failure or from an investigation under the National Construction Safety Team Act may be used in any suit or action for damages arising out of any matter mentioned in such report (15 USC 281a; as amended by P.L 107-231)

National Institute of Standards and Technology National Construction Safety Team Act Report 1-3

Natl Inst Stand Technol Natl Constr Sfty Tm Act Rpt 1-3, 184 pages (September 2005)

CODEN: NSPUE2

U.S GOVERNMENT PRINTING OFFICE

WASHINGTON: 2005

_

A BSTRACT

This report is an overview of the results of the mechanical and metallurgical analysis of structural steel from the World Trade Center (WTC), part of the National Institute of Standards and Technology

Investigation of the WTC disaster of September 11, 2001

The goal of the study was threefold:

Determine mechanical properties of WTC structural steel,

Determine the quality of the steel and if design requirements were met, and

Analyze the recovered steel to provide insight into failure mechanisms to guide and/or validate models of building performance

Structural steel recovered from the WTC site was analyzed for composition, microstructure, and

mechanical properties, including room temperature properties (for modeling baseline building

performance), high temperature properties (for modeling structural response of the building to fire), and behavior at high strain rates (for modeling airplane impact) Failure analysis of the recovered steel, complemented by pre-collapse photographs of the damaged building, was used to establish failure modes and temperature excursions experienced by the steel In addition, documents from the construction era covering issues ranging from steel specifications to engineering design drawings were used to help interpret the results and supplement models of mechanical properties used in the models of building performance

The analysis focused on the WTC 1 and WTC 2 Although no steel was recovered from WTC 7, a

47-story building that also collapsed on September 11, properties for steel used in its construction were estimated based on literature and contemporaneous documents

Keywords: Failure analysis, mechanical properties, specifications, structural steel, World Trade Center

Abstract

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T ABLE OF C ONTENTS

Abstract iii

List of Figures xi

List of Tables xvii

List of Acronyms and Abbreviations xix

Metric Conversion Table xxi

Preface xxv

Acknowledgments xxxv

Executive Summary xxxvii

Chapter 1 Introduction 1

1.1 Overview and Scope 1

1.2 Approach 2

Chapter 2 Tower Design – Structural Steel Documents 5

2.1 Introduction 5

2.2 Specification of Steel Grades (Minimum Yield Strength) 5

2.3 Structural Overview 5

2.3.1 Perimeter Columns 7

2.3.2 Core Columns 10

2.3.3 Flooring System 10

2.3.4 Floors 107 to 110 13

2.3.5 Impact Zone 14

2.3.6 Floors Involved in Post-Impact Fires 14

2.4 References 14

Chapter 3 Contemporaneous Steel Specifications 15

3.1 Introduction 15

3.2 Floor Trusses 15

3.3 Perimeter Columns and Spandrels 16

3.4 Core—Welded Box Columns 17

Table of Contents

3.5 Core—Rolled Wide flange Shapes 17

3.6 Findings 18

3.7 References 18

Chapter 4 Contemporaneous Construction Specifications 21

4.1 Introduction 21

4.2 Fabrication of the Various Components 21

4.2.1 Floor Trusses 21

4.2.2 Exterior Wall Columns and Spandrels 21

4.2.3 Core—Welded Box Columns 22

4.2.4 Connections (Bolts and Welds) 23

4.2.5 Construction (On-Site Assembly) 23

4.3 References 24

Chapter 5 Steel Inventory and Identification 27

5.1 The Recovery of World Trade Center Structural Steel 27

5.2 Structural Elements Recovered from WTC Buildings 28

5.2.1 Location and Labeling of Structural Steel Elements 28

5.2.2 Identification Methods of WTC Structural Steel Elements 29

5.2.3 Identified Perimeter Panels 33

5.2.4 Identified Core Columns 35

5.2.5 Other Structural Pieces 36

5.3 Structural Steel Elements of Special Importance 36

5.3.1 Samples Located in or around the Floors of the Airplane Impact 36

5.3.2 Samples Representing the Various Types of Steel Specified in the Design Drawings 38

5.4 Summary 39

5.5 References 39

Chapter 6 Damage and Failure Analysis of Structural Steel 41

6.1 Background 41

6.2 Observable Pre-Collapse Damage of Exterior Column Panels 42

6.2.1 Photographic Analysis of Pre-Collapse Damage to Exterior Panels 42

6.2.2 Photographic Evidence of Damage to the Flooring System of WTC 2 45

Table of Contents

6.2.4 Damage to Sprayed Fire-Resistive Material in WTC 2 49

6.2.5 Photographic Analysis of Pre-Collapse Fire Exposure to Exterior Panels 55

6.2.6 Photographic Evidence of Pre-Collapse Distortion of Exterior Wall: South Face of WTC 1 59

6.2.7 Photographic Evidence of Pre-Collapse Distortion of Exterior Walls: East Face of WTC 2 63

6.2.8 Photographic Evidence of Details of the Collapse of WTC 2 63

6.3 Physical Damage of Recovered Exterior Wall Panels 69

6.3.1 Overall Damage Patterns of Exterior Wall Panel Sections 70

6.3.2 Damage and Failure Modes of Exterior Wall Columns 70

6.3.3 Exterior Wall Spandrel Connections 74

6.3.4 Exterior Wall Column Splices (End Plate/Butt Plate Connections) 74

6.3.5 Exterior Wall Seats or Floor Truss Connectors 80

6.4 Physical Damage of Core Elements (Columns and Channels) 80

6.4.1 Core Columns 80

6.4.2 Core Channels and Truss Seats 92

6.5 Physical Damage of Floor Truss Material 93

6.6 Fire Exposure of the Structural Elements 94

6.6.1 Visual Inspection of Recovered Structural Components 94

6.6.2 Core Columns Exposed to Fire 95

6.6.3 Metallographic Analysis of Elements Exposed to Fire 95

6.7 Comments Concerning Contract to Wiss, Janney, Elstner 99

6.8 Findings 99

6.8.1 Structural Damage of Exterior Panel Sections 99

6.8.2 Structural Damage of Core Columns 100

6.8.3 Structural Damage of Floor Trusses and Seats 100

6.8.4 Damage to Sprayed Fire-Resistive Material 100

6.8.5 Fire Exposure of Exterior Panel Sections 101

6.8.6 Fire Exposure of Core Columns 101

6.8.7 Time-Dependent Deformation of Perimeter Walls due to Fire and Load Redistribution 101

6.8.8 Other 102

Chapter 7 Mechanical Properties of Structural Steels 103

7.1 Introduction 103

Table of Contents

7.2 Elastic Modulus 103

7.3 Room Temperature Tensile Properties 104

7.3.1 Measured Tensile Properties 104

7.3.2 Measured Properties and Specifications 104

7.3.3 Representative Steel Stress-Strain Curves 107

7.4 High-Strain Rate Tests 108

7.5 Charpy Tests 109

7.6 High-Temperature Tests 111

7.7 Steels Used in WTC 7 113

7.7.1 Steels Used in Construction of WTC 7 113

7.7.2 Mechanical Properties of WTC 7 Steel 114

7.7.3 Physical Properties of WTC 7 Steel 115

7.8 Findings 115

7.8.1 Room Temperature Tensile Properties 115

7.8.2 High-Strain Rate Properties 116

7.8.3 Impact Properties 116

7.8.4 High-Temperature Properties 116

7.9 References 116

Chapter 8 Physical Properties of Structural Steels 119

8.1 Introduction 119

8.2 Chemical Analysis of Recovered Steel 119

8.3 Metallography 120

8.4 Thermal Properties 122

8.5 Summary 124

8.5.1 Chemistry of WTC Steels 124

8.5.2 Microstructure of WTC Steels 125

8.5.3 Thermal Property Values of WTC Steels 126

8.5.4 Other 126

8.6 References 126

Chapter 9 Findings and Issues 129

9.1 Introduction 129

9.2 Findings – Steel Specifications, Standards, and Sources 129

Table of Contents

9.3 Findings – Inventory of Recovered Structural Steel 129

9.4 Findings – Damage and Failure Analysis 130

9.4.1 Structural Impact Damage – Perimeter Panels 130

9.4.2 Structural Impact Damage – Core Columns 131

9.4.3 Impact Damage to Sprayed Fire-Resistive Material 131

9.4.4 Structural Damage – Floor Trusses and Seats 131

9.4.5 Fire Exposure and Temperatures Reached by the Steel 132

9.4.6 Time-Dependent Deformation of Perimeter Walls Due to Fire and Load Redistribution 132

9.5 Findings – Mechanical Properties 133

9.5.1 Room Temperature Tensile Properties 133

9.5.2 High-Strain-Rate Properties 134

9.5.3 Impact Properties 134

9.5.4 High-Temperature Properties 134

9.5.5 Conclusions – Mechanical Properties 134

9.6 Issues 134

Table of Contents

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L IST OF F IGURES

Figure P–1 The eight projects in the federal building and fire safety investigation of the WTC

disaster xxvii

Figure 2–1 Schematic diagram of the tower structure 6

Figure 2–2 WTC 1 tower floor plan and column numbers 7

Figure 2–3 Cross-section of perimeter columns; sections with and without spandrels 8

Figure 2–4 Characteristic perimeter column panel consisting of three full columns connected by three spandrels 8

Figure 2–5 Partial elevation of exterior bearing-wall frame showing exterior wall module construction Highlighted panel is three stories tall (36 ft) and spans four floors Distance between panels has been exaggerated 9

Figure 2–6 Core column layout in WTC towers 10

Figure 2–7 Typical welded box members and rolled wide flange shapes used for core columns between floors 83 and 86 11

Figure 2–8 Schematic diagram of a floor truss 12

Figure 2–9 Hat truss in upper floors 13

Figure 5–1 Example of stampings on the interior base of the middle column for each panel Shown here on panel M-10b are the stampings “B206 86 83,” indicating that it is from WTC 2, column number 206, spanning floors 83 to 86 30

Figure 5–2 Example of stampings placed on one end of a core column Shown here on sample C-90 are the stampings “18 TON, 701 B 12 – 16, 549 WEST,” indicating that it is from WTC 2, core column number 701, and spans floors 12 to 16 Also indicated are the approximate weight of the piece (18 tons) and the derrick division <549> that was to lift it into place 31

Figure 5–3 a) and b) Characteristic stenciling found on the lower portions of the exterior column panels for sample M-27 Markings indicate this piece was in WTC 1, column number 130, spanning floors 93 to 96 Also seen are two other indicators, “5T” and “<63>,” indicating the estimated piece weight in tons and erector’s derrick division number, respectively c) Characteristic stenciling found on core column for sample B-6152-1 Markings indicate this piece was in WTC 1, column number 803, spanning floors 15 to 18 with a derrick division number of <52> 32

Figure 5–4 Location of the exterior panels recovered from the top third of the WTC towers Recovered WTC 1 panels are shown in light shading/green and WTC 2 panels are in dark shading/blue 34

List of Figures

Figure 5–5a Interpreted column damage, from photographic evidence, to north face of WTC 1,

with overlay of recovered steel identified by NIST Colored regions represent

recovered portions Both core columns (#603 and #605) are in the second row from

the north face of WTC 1 37Figure 5–5b Interpreted column damage, from photographic evidence, to south face of WTC 2,

with overlay of recovered steel identified by NIST Samples shown represent

recovered portions Core column line 801 is in the row closest to the south face of

WTC 2 38

Figure 6–1 (top) Enhanced image showing the impact hole in the north face of WTC 1, and

(bottom) overlay of an outline of a Boeing 767 that has been distorted to get a best

approximate fit with damage to WTC 1 The location and type of localized damage

has been indicated 43Figure 6–2 (top) Enhanced image showing the impact hole in the south face of WTC 2 and

(bottom) overlay of the location and type of localized damage 44Figure 6–3 Damage to northeast corner of WTC 2 at 81st floor The red arrow indicates an

intact column 300 at the 82nd floor Other photographs confirm that the spandrel

connecting column 300 to columns 259 and 301 is also intact (location indicated by

green arrows) 46Figure 6–4 Damage to columns on north face of WTC 2 near northeast corner due to internal

impact by debris Broken vertical column bolted connections (green arrows) are

visible, along with an intact outer web on column 254 (red arrows) and missing

column sections (blue arrows) 47Figure 6–5 Dropped floors on north face of WTC 2 48Figure 6–6 Diagrams showing how the fireproofing was to be applied to the exterior columns,

and the exterior aluminum panels were attached to the window frames 50Figure 6–7 Fireproofing that had been applied to the exterior columns The depression formed

by the outer web and the flange tips are completely filled in with fireproofing at the

arrow North face of WTC 2 51Figure 6–8 Port side of impact hole in north face of WTC 1 showing missing fireproofing The

red arrows indicate a few of the many instances of missing fireproofing around the

impact hole The region roughly approximated as the area where the aluminum

panels were dislodge has been digitally enhanced for clarity 52Figure 6–9 Missing fireproofing on floor trusses that fell into impact hole on north face of WTC

1 The arrows labeled “1” are three trusses where the lower or upper chords can be

seen, and can distinguish the line that separates the two angle iron components The

arrows labeled “2” lie at either end of a lower chord of a truss that fell farther inside

the building, and is made up of four angle irons (a double truss) The red paint color being visible and the fact that the line between the angle iron can be seen indicated

that the fireproofing was displaced 53

List of Figures

Figure 6–10 Missing façade and fireproofing on north face of WTC 2 Two regions have had

contrast and gamma adjusted to enhance visibility of the features Red arrows

indicate columns with missing or damaged fireproofing Missing or damaged

fireproofing determinations were made from this and several other photos taken at

different times and angles Blue arrows indicate white features where some sort of

coating has preserved the fireproofing in place 54Figure 6–11 Missing fireproofing on flanges of exterior columns of north face of WTC 2

Fireproofing has come off, revealing red Tnemec paint of the column (red arrows)

This image also shows how the outer web regions have been partially or completely

denuded of fireproofing Damage is evidenced by shading by sunlight - compare

green (undamaged fireproofing) and white (damaged fireproofing) arrows Blue

arrow indicates same type of white coated area of fireproofing as previous figure that adhered when material above and below was dislodged 56Figure 6–12 Damage to fireproofing on east face of WTC 2 due to internal impact Red arrows

highlight areas where fireproofing has been damaged The blowup to the right shows

a column where the red Tnemec paint of the column is visible 57Figure 6–13 Time-fire exposure map for panel K-2 (A236: 92-95) Fire can be observed on the

92nd and 94th floors of the panel 58Figure 6–14 Inward bowing of south face (at right, with dark lines overdrawn) of WTC 1 Taken

at 10:23 a.m from an NYPD helicopter 61Figure 6–15 Pull in of south face of WTC 1 at 10:23 a.m showing considerable displacements of

outer columns into the building Faint white vertical features are the aluminum

façade of the exterior Maximum visible inward displacement of the columns is 55

in +/- 6 in Poor resolution leads to large uncertainties in measurements; a large

fraction of the south face of the building is obscured by smoke 62Figure 6–16 East face of WTC 2 at 9:53:04 a.m showing a larger amount of pull in The vertical

black lines were drawn to establish the original line of the exterior wall columns, and the shorter line segments are drawn at the same angle as the nearest observable joint

in the exterior aluminum panels to establish the inward bowing distance For actual

column numbers, add a “3” to the front of the two-digit designations in the image 64Figure 6–17 Map of inward displacements of the east outer face of WTC 2 inward at 9:21:29 a.m

Each measurement refers to the upper left corner of the window within which the

number resides Measurements were in inches The colors refer to groups of 5 in

displacements (0–5: black, 6–10: blue) for easier visualization 65Figure 6–18 Map of pull in displacements of the east outer face of WTC 2 inward shortly before

collapse Each measurement refers to the upper left corner of the window within

which the number resides Measurements were in inches The colors refer to groups

of 5 in displacements (0–5: black, 6–10: blue, 11–15: orange, 16–20: red) for easier

visualization This data was combined from two images taken by the same

photographer at nearly the same location 10 s apart, and it is assumed that the inward deflection did not change appreciably during this time 66Figure 6–19 Initial seconds of collapse of WTC 2 viewed from the east-northeast, and kink in the

southeast edge of the building near the 106th floor (arrow) 67

List of Figures

Figure 6–20 A close-up of the kink area in the previous figure (southeast corner, WTC 2), with

overlaid lines to highlight the details of the kink geometry The edge of the building (1) bends an angle to another direction (2), then bends back somewhat toward the

original direction (3) 68Figure 6–21 Northeast corner of WTC 2, several seconds before collapse The corner shows no

distortion of the type in the previous figure, implying that the distortion accompanied the collapse and did not precede it The region in the upper right is enhanced for

visibility 69Figure 6–22 a) Overall view of panel M-2 (A130: 94-97) Bottom of panel is on the left-hand side

of the picture and the outside of the panel is facing down, b) major bend in panel

located at the 97th floor level, and c) splayed bottoms of columns 129 and 130 71Figure 6–23 Thinning of outer web from column 130 of panel M-2 (A130: 96-99) a) Photograph

showing thinning, b) graph indicating plate thinning near fracture surface 72Figure 6–24 Damage images from sample M-2 (A130: 96-99) a) Large buckles observed on the

flange plates of column 130, b) cracking of the flange plates on column 130,

c) cracking of spandrel at 97th floor level, and d) no cracking of the welds in this

area on column 130 73Figure 6–25 Photographs of recovered spandrel connections from sample M-2 (A130: 96-99) 75Figure 6–26 Photographs of recovered end plate connections from sample M-2 (A127: 97-100)

Both ends of column 131 had bolt failure that resulted in deformation of the bolt

holes in the end plates 77Figure 6–27 Floor truss connectors for panel M-2 (A130: 96-99) 81Figure 6–28a Damage diagram overlaid with recovered samples and damage of connectors 84Figure 6–28b Damage of connectors At or below the 95th floor, all connectors are either bent

down or missing 85Figure 6–29a Positioning of the recovered core columns with the core area for WTC 1 86Figure 6–29b Positioning of the recovered core columns with the core area for WTC 2 87Figure 6–30 Core column C-80 (603A: 92-95) a) Overall view of recovered column, b) view of

connection in the 92nd floor region, failure as a result of bolt fracture, and c) torn end

of column near the 94th floor region 88Figure 6–31 Overall view of recovered core columns C-88a (801B: 80-83) and C-88b (801B:

77-80) 89Figure 6–32 Damage associated with core column C-88b (801B: 77-80) a) Overall view of failed

end, b) bottoms of “south” flange and both webs bent towards the east just below the 80th floor level, ends were flame cut, and c) “north” flange bent towards the north,

majority of plate was fractured with some flame cutting 90Figure 6–33 Core column HH (605A: 98-101) a) Fracture at both ends and b) separation between

flange and web in the 99th floor region 91Figure 6–34 Failure modes of core channels a) Fracture through channel as indicated by arrows,

b) failure associated with end connector, and c) seat ripped off at welded connection

to channel 92Figure 6–35 Examples of recovered floor truss material a) Small sections of rod and chord

List of Figures

Figure 6–36 Crushed portion of column 210 in the area of the 98th floor (from sample K-1, A209:

97-100) Paint had mud cracking characteristic in this region 96Figure 6–37 Spandrel at the 93rd floor of column 236 from sample K-2 (A236: 92-95) Paint mud

cracking was found below the 93rd floor line, no evidence of paint mud cracking was found above floor line 97Figure 6–38 Truss seat of the 99th floor on column 143 of sample N-8 (A142: 97-100) a) Top

view showing the re-solidified black plastic with embedded papers, most likely a binder of some type and b) bottom view showing the drip pattern It was unknown when the high-temperature excursion may have occurred 98

Figure 7–1 Elastic modulus as a function of temperature determined by NIST compared with

literature data for structural steel 104Figure 7–2 Examples of longitudinal (l1, l2, and l3) and transverse (t1) room-temperature, stress-

strain curves for a F y =46 ksi perimeter column spandrel plate 105Figure 7–3 Ratio of measured yield strength or yield point to specified minimum yield point for

WTC perimeter column steels 105Figure 7–4 Ratio of measured yield strength or yield point to specified minimum yield point for

WTC core column steels 106Figure 7–5 Examples of representative true-stress true-strain curves for perimeter column steels 108Figure 7–6 Strain rate sensitivity as a function of specified minimum yield strength for WTC

steels 109Figure 7–7 Summary data for Charpy tests Lower figure: perimeter column steels; upper figure:

core column steels 110Figure 7–8 Normalized yield strength as a function of temperature for WTC steels, the

recommended function to describe behavior (solid line), and the literature data used

to develop that relation 111Figure 7–9 Normalized tensile strength as a function of temperature for WTC steels, the

recommended function to describe behavior (solid line), and the literature data used

to develop that relation 112

Figure 7–10 Typical high-temperature stress-strain curves for specimen HH, a F y = 42 ksi

wide-flange shape from near the fire and impact zone of WTC 1 113Figure 7–11 Typical creep curves for different stresses at 600 °C from truss steels specified as

A 242, from specimen C132-TA 114

Figure 8–1 Microstructure of ferritic-pearlitic WTC perimeter column flange plate steels a) F y =

45 ksi (unidentified perimeter column with “45” flange stamping) and b) F y = 60 ksi (WTC 1, column 126, 97th floor) 121

Figure 8–2 Examples of “quenched and tempered” WTC perimeter column flange steel a) F y =

70 ksi (WTC 1, column 224, 94th floor) and b) F y = 80 ksi (WTC 1, column 207, 89th floor) 122Figure 8–3 Thermal conductivity as a function of temperature for twelve low-alloy steels 125

List of Figures

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L IST OF T ABLES

Table P–1 Federal building and fire safety investigation of the WTC disaster xxvi

Table P–2 Public meetings and briefings of the WTC Investigation xxix

Table 3–1 Steel companies involved in WTC construction and their contracts 16

Table 5–1 Identified perimeter column panel pieces from WTC 1 and WTC 2 33

Table 5–2 Identified pieces of core column material from WTC 1 and WTC 2 35

Table 6–1 Recovered exterior panel sections with known as-built locations, separated by pre-collapse location and environmental conditions 60

Table 6–2 Recovered core columns with known as-built locations, separated by pre-collapse conditions 86

List of Tables

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L IST OF A CRONYMS AND A BBREVIATIONS

Acronyms

AISC American Institute of Steel Construction

ASCE American Society of Civil Engineers

ASTM ASTM International

AWS American Welding Society

CTE coefficient of thermal expansion

DTAP dissemination and technical assistance program

FEMA Federal Emergency Management Agency

GMS Gilsanz Murray Steficek, LLP

HAZ heat affected zone

HSLA high-strength, low-alloy

JFK John F Kennedy International Airport

LERA Leslie E Robertson Associates

NCST National Construction Safety Team

NIST National Institute of Standards and Technology

NYPD New York City Police Department

PANYNJ Port Authority of New York and New Jersey

PC&F Pacific Car and Foundry

PONYA Port of New York Authority

R&D research and development

SEAoNY Structural Engineers Association of New York

SFRM sprayed fire-resistive materials

SHCR Skilling, Helle, Christiansen, & Robertson

SMA shielded metal arc

USC United States Code

WJE Wiss, Janney, Elstner Associates, Inc

WTC World Trade Center

WTC 1 World Trade Center 1 (North Tower)

List of Acronyms and Abbreviations

WTC 2 World Trade Center 2 (South Tower)

WTC 7 World Trade Center 7

kip a force equal to 1,000 pounds

ksi 1,000 pounds per square inch

M ETRIC C ONVERSION T ABLE

To convert from to Multiply by

AREA AND SECOND MOMENT OF AREA

square foot (ft2) square meter (m2) 9.290 304 E-02

square inch (in.2) square meter (m2) 6.4516 E-04

square inch (in.2) square centimeter (cm2) 6.4516 E+00

FORCE

FORCE DIVIDED BY LENGTH

pound-force per foot (lbf/ft) newton per meter (N/m) 1.459 390 E+01

pound-force per inch (lbf/in.) newton per meter (N/m) 1.751 268 E+02

LENGTH

MASS and MOMENT OF INERTIA

kilogram-force second squared

per meter (kgf ⋅ s 2

pound foot squared (lb ⋅ ft 2

) kilogram meter squared (kg ⋅ m 2

) 4.214 011 E-02 pound inch squared (lb ⋅ in 2 ) kilogram meter squared (kg ⋅ m 2 ) 2.926 397 E-04

MASS DIVIDED BY AREA

pound per square foot (lb/ft2) kilogram per square meter (kg/m2) 4.882 428 E+00

pound per square inch

(not pound force) (lb/in.2) kilogram per square meter (kg/m2) 7.030 696 E+02

Metric Conversion Table

To convert from to Multiply by

MASS DIVIDED BY LENGTH

pound per foot (lb/ft) kilogram per meter (kg/m) 1.488 164 E+00

pound per inch (lb/in.) kilogram per meter (kg/m) 1.785 797 E+01

pound per yard (lb/yd) kilogram per meter (kg/m) 4.960 546 E-01

PRESSURE or STRESS (FORCE DIVIDED BY AREA)

kilogram-force per square centimeter (kgf/cm 2 ) pascal (Pa) 9.806 65 E+04

kilogram-force per square meter (kgf/m2) pascal (Pa) 9.806 65 E+00

kilogram-force per square millimeter (kgf/mm 2 ) pascal (Pa) 9.806 65 E+06

kip per square inch (ksi) (kip/in.2) pascal (Pa) 6.894 757 E+06

kip per square inch (ksi) (kip/in.2) kilopascal (kPa) 6.894 757 E+03

pound-force per square foot (lbf/ft2) pascal (Pa) 4.788 026 E+01

pound-force per square inch (psi) (lbf/in.2) pascal (Pa) 6.894 757 E+03

pound-force per square inch (psi) (lbf/in.2) kilopascal (kPa) 6.894 757 E+00

psi (pound-force per square inch) (lbf/in.2) pascal (Pa) 6.894 757 E+03

psi (pound-force per square inch) (lbf/in.2) kilopascal (kPa) 6.894 757 E+00

TEMPERATURE

degree centigrade degree Celsius (°C) t/°C ≈ t /deg cent

degree Fahrenheit (°F) degree Celsius (°C) t/°C = (t/°F - 32)/1.8

degree Fahrenheit (°F) kelvin (K) T/K = (t/°F + 459.67)/1.8

kelvin (K) degree Celsius (°C) t/°C = T/K 2 273.15

TEMPERATURE INTERVAL

degree Fahrenheit (°F) degree Celsius (°C) 5.555 556 E-01

Metric Conversion Table

To convert from to Multiply by

VELOCITY (includes SPEED)

foot per second (ft/s) meter per second (m/s) 3.048 E-01

inch per second (in./s) meter per second (m/s) 2.54 E-02

kilometer per hour (km/h) meter per second (m/s) 2.777 778 E-01

mile per hour (mi/h) kilometer per hour (km/h) 1.609 344 E+00

mile per minute (mi/min) meter per second (m/s) 2.682 24 E+01

VOLUME (includes CAPACITY)

gallon (U.S.) (gal) cubic meter (m3) 3.785 412 E-03

ounce (U.S fluid) (fl oz) cubic meter (m3) 2.957 353 E-05

Metric Conversion Table

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P REFACE

Genesis of This Investigation

Immediately following the terrorist attack on the World Trade Center (WTC) on September 11, 2001, the Federal Emergency Management Agency (FEMA) and the American Society of Civil Engineers began planning a building performance study of the disaster The week of October 7, as soon as the rescue and search efforts ceased, the Building Performance Study Team went to the site and began its assessment This was to be a brief effort, as the study team consisted of experts who largely volunteered their time away from their other professional commitments The Building Performance Study Team issued its report in May 2002, fulfilling its goal “to determine probable failure mechanisms and to identify areas of future investigation that could lead to practical measures for improving the damage resistance of buildings against such unforeseen events.”

On August 21, 2002, with funding from the U.S Congress through FEMA, the National Institute of Standards and Technology (NIST) announced its building and fire safety investigation of the WTC disaster On October 1, 2002, the National Construction Safety Team Act (Public Law 107-231), was signed into law The NIST WTC Investigation was conducted under the authority of the National

Construction Safety Team Act

The goals of the investigation of the WTC disaster were:

• To investigate the building construction, the materials used, and the technical conditions that contributed to the outcome of the WTC disaster

• To serve as the basis for:

− Improvements in the way buildings are designed, constructed, maintained, and used;

− Improved tools and guidance for industry and safety officials;

− Recommended revisions to current codes, standards, and practices; and

− Improved public safety

The specific objectives were:

1 Determine why and how WTC 1 and WTC 2 collapsed following the initial impacts of the aircraft and why and how WTC 7 collapsed;

2 Determine why the injuries and fatalities were so high or low depending on location,

including all technical aspects of fire protection, occupant behavior, evacuation, and

Preface

NIST is a nonregulatory agency of the U.S Department of Commerce’s Technology Administration The

purpose of NIST investigations is to improve the safety and structural integrity of buildings in the United

States, and the focus is on fact finding NIST investigative teams are authorized to assess building

performance and emergency response and evacuation procedures in the wake of any building failure that

has resulted in substantial loss of life or that posed significant potential of substantial loss of life NIST

does not have the statutory authority to make findings of fault nor negligence by individuals or

organizations Further, no part of any report resulting from a NIST investigation into a building failure or

from an investigation under the National Construction Safety Team Act may be used in any suit or action

for damages arising out of any matter mentioned in such report (15 USC 281a, as amended by Public

Law 107-231)

Organization of the Investigation

The National Construction Safety Team for this Investigation, appointed by the then NIST Director,

Dr Arden L Bement, Jr., was led by Dr S Shyam Sunder Dr William L Grosshandler served as

Associate Lead Investigator, Mr Stephen A Cauffman served as Program Manager for Administration,

and Mr Harold E Nelson served on the team as a private sector expert The Investigation included eight

interdependent projects whose leaders comprised the remainder of the team A detailed description of

each of these eight projects is available at http://wtc.nist.gov The purpose of each project is summarized

in Table P–1, and the key interdependencies among the projects are illustrated in Fig P–1

Table P–1 Federal building and fire safety investigation of the WTC disaster

Analysis of Building and Fire Codes and

Practices; Project Leaders: Dr H S Lew

and Mr Richard W Bukowski

Document and analyze the code provisions, procedures, and practices used in the design, construction, operation, and maintenance of the structural, passive fire protection, and emergency access and evacuation systems of WTC 1, 2, and 7

Baseline Structural Performance and

Aircraft Impact Damage Analysis; Project

Leader: Dr Fahim H Sadek

Analyze the baseline performance of WTC 1 and WTC 2 under design, service, and abnormal loads, and aircraft impact damage on the structural, fire protection, and egress systems

Mechanical and Metallurgical Analysis of

Structural Steel; Project Leader: Dr Frank

W Gayle

Determine and analyze the mechanical and metallurgical properties and quality of steel, weldments, and connections from steel recovered from WTC 1, 2, and 7

Investigation of Active Fire Protection

Systems; Project Leader: Dr David

D Evans; Dr William Grosshandler

Investigate the performance of the active fire protection systems in WTC 1, 2, and 7 and their role in fire control, emergency response, and fate of occupants and responders

Reconstruction of Thermal and Tenability

Environment; Project Leader: Dr Richard

G Gann

Reconstruct the time-evolving temperature, thermal environment, and smoke movement in WTC 1, 2, and 7 for use in evaluating the structural performance of the buildings and behavior and fate of occupants and responders

Structural Fire Response and Collapse

Analysis; Project Leaders: Dr John

L Gross and Dr Therese P McAllister

Analyze the response of the WTC towers to fires with and without aircraft damage, the response of WTC 7 in fires, the performance

of composite steel-trussed floor systems, and determine the most probable structural collapse sequence for WTC 1, 2, and 7

Occupant Behavior, Egress, and Emergency

Communications; Project Leader: Mr Jason

D Averill

Analyze the behavior and fate of occupants and responders, both those who survived and those who did not, and the performance of the evacuation system

Emergency Response Technologies and

Guidelines; Project Leader: Mr J Randall

Document the activities of the emergency responders from the time

of the terrorist attacks on WTC 1 and WTC 2 until the collapse of

Preface

NIST WTC Investigation Projects

Analysis of Steel Structural Collapse

Evacuation

Baseline Performance

& Impact Damage

Analysis of Codes and Practices

Emergency Response

Active Fire Protection

Thermal and Tenability Environment

Figure P–1 The eight projects in the federal building and fire safety

investigation of the WTC disaster

National Construction Safety Team Advisory Committee

The NIST Director also established an advisory committee as mandated under the National Construction Safety Team Act The initial members of the committee were appointed following a public solicitation These were:

• Paul Fitzgerald, Executive Vice President (retired) FM Global, National Construction Safety Team Advisory Committee Chair

• John Barsom, President, Barsom Consulting, Ltd

• John Bryan, Professor Emeritus, University of Maryland

• David Collins, President, The Preview Group, Inc

• Glenn Corbett, Professor, John Jay College of Criminal Justice

• Philip DiNenno, President, Hughes Associates, Inc

Preface

• Robert Hanson, Professor Emeritus, University of Michigan

• Charles Thornton, Co-Chairman and Managing Principal, The Thornton-Tomasetti Group,

Inc

• Kathleen Tierney, Director, Natural Hazards Research and Applications Information Center,

University of Colorado at Boulder

• Forman Williams, Director, Center for Energy Research, University of California at San

Diego

This National Construction Safety Team Advisory Committee provided technical advice during the

Investigation and commentary on drafts of the Investigation reports prior to their public release NIST

has benefited from the work of many people in the preparation of these reports, including the National

Construction Safety Team Advisory Committee The content of the reports and recommendations,

however, are solely the responsibility of NIST

Public Outreach

During the course of this Investigation, NIST held public briefings and meetings (listed in Table P–2) to

solicit input from the public, present preliminary findings, and obtain comments on the direction and

progress of the Investigation from the public and the Advisory Committee

NIST maintained a publicly accessible Web site during this Investigation at http://wtc.nist.gov The site

contained extensive information on the background and progress of the Investigation

NIST’s WTC Public-Private Response Plan

The collapse of the WTC buildings has led to broad reexamination of how tall buildings are designed,

constructed, maintained, and used, especially with regard to major events such as fires, natural disasters,

and terrorist attacks Reflecting the enhanced interest in effecting necessary change, NIST, with support

from Congress and the Administration, has put in place a program, the goal of which is to develop and

implement the standards, technology, and practices needed for cost-effective improvements to the safety

and security of buildings and building occupants, including evacuation, emergency response procedures,

and threat mitigation

The strategy to meet this goal is a three-part NIST-led public-private response program that includes:

• A federal building and fire safety investigation to study the most probable factors that

contributed to post-aircraft impact collapse of the WTC towers and the 47-story WTC 7

building, and the associated evacuation and emergency response experience

• A research and development (R&D) program to (a) facilitate the implementation of

recommendations resulting from the WTC Investigation, and (b) provide the technical basis

for cost-effective improvements to national building and fire codes, standards, and practices

that enhance the safety of buildings, their occupants, and emergency responders

Preface

Table P–2 Public meetings and briefings of the WTC Investigation

June 24, 2002 New York City, NY Public meeting: Public comments on the Draft Plan for the

pending WTC Investigation

August 21, 2002 Gaithersburg, MD Media briefing announcing the formal start of the Investigation December 9, 2002 Washington, DC Media briefing on release of the Public Update and NIST request

for photographs and videos

April 8, 2003 New York City, NY Joint public forum with Columbia University on first-person

interviews

April 29–30, 2003 Gaithersburg, MD NCST Advisory Committee meeting on plan for and progress on

WTC Investigation with a public comment session

May 7, 2003 New York City, NY Media briefing on release of May 2003 Progress Report

August 26–27, 2003 Gaithersburg, MD NCST Advisory Committee meeting on status of the WTC

investigation with a public comment session

September 17, 2003 New York City, NY Media and public briefing on initiation of first-person data

collection projects

December 2–3, 2003 Gaithersburg, MD NCST Advisory Committee meeting on status and initial results

and release of the Public Update with a public comment session

February 12, 2004 New York City, NY Public meeting on progress and preliminary findings with public

comments on issues to be considered in formulating final recommendations

June 18, 2004 New York City, NY Media/public briefing on release of June 2004 Progress Report

June 22–23, 2004 Gaithersburg, MD NCST Advisory Committee meeting on the status of and

preliminary findings from the WTC Investigation with a public comment session

August 24, 2004 Northbrook, IL Public viewing of standard fire resistance test of WTC floor

system at Underwriters Laboratories, Inc

October 19–20, 2004 Gaithersburg, MD NCST Advisory Committee meeting on status and near complete

set of preliminary findings with a public comment session

November 22, 2004 Gaithersburg, MD NCST Advisory Committee discussion on draft annual report to

Congress, a public comment session, and a closed session to discuss pre-draft recommendations for WTC Investigation

April 5, 2005 New York City, NY Media and public briefing on release of the probable collapse

sequence for the WTC towers and draft reports for the projects on codes and practices, evacuation, and emergency response

June 23, 2005 New York City, NY Media and public briefing on release of all draft reports for the

WTC towers and draft recommendations for public comment September 12–13,

2005

Gaithersburg, MD NCST Advisory Committee meeting on disposition of public

comments and update to draft reports for the WTC towers

September 13–15,

2005

Gaithersburg, MD WTC Technical Conference for stakeholders and technical

community for dissemination of findings and recommendations and opportunity for public to make technical comments

• A dissemination and technical assistance program (DTAP) to (a) engage leaders of the construction and building community in ensuring timely adoption and widespread use of proposed changes to practices, standards, and codes resulting from the WTC Investigation and the R&D program, and (b) provide practical guidance and tools to better prepare facility owners, contractors, architects, engineers, emergency responders, and regulatory authorities

to respond to future disasters

The desired outcomes are to make buildings, occupants, and first responders safer in future disaster events

Preface

National Construction Safety Team Reports on the WTC Investigation

A final report on the collapse of the WTC towers is being issued as NIST NCSTAR 1 A companion

report on the collapse of WTC 7 is being issued as NIST NCSTAR 1A The present report is one of a set

that provides more detailed documentation of the Investigation findings and the means by which these

technical results were achieved As such, it is part of the archival record of this Investigation The titles

of the full set of Investigation publications are:

NIST (National Institute of Standards and Technology) 2005 Federal Building and Fire Safety

Investigation of the World Trade Center Disaster: Final Report on the Collapse of the World Trade

Center Towers NIST NCSTAR 1 Gaithersburg, MD, September

NIST (National Institute of Standards and Technology) 2006 Federal Building and Fire Safety

Investigation of the World Trade Center Disaster: Final Report on the Collapse of World Trade Center 7

NIST NCSTAR 1A Gaithersburg, MD

Lew, H S., R W Bukowski, and N J Carino 2005 Federal Building and Fire Safety Investigation of

the World Trade Center Disaster: Design, Construction, and Maintenance of Structural and Life Safety

Systems NIST NCSTAR 1-1 National Institute of Standards and Technology Gaithersburg, MD,

September

Fanella, D A., A T Derecho, and S K Ghosh 2005 Federal Building and Fire Safety

Investigation of the World Trade Center Disaster: Design and Construction of Structural Systems

NIST NCSTAR 1-1A National Institute of Standards and Technology Gaithersburg, MD,

September

Ghosh, S K., and X Liang 2005 Federal Building and Fire Safety Investigation of the World

Trade Center Disaster: Comparison of Building Code Structural Requirements NIST

NCSTAR 1-1B National Institute of Standards and Technology Gaithersburg, MD, September

Fanella, D A., A T Derecho, and S K Ghosh 2005 Federal Building and Fire Safety

Investigation of the World Trade Center Disaster: Maintenance and Modifications to Structural

Systems NIST NCSTAR 1-1C National Institute of Standards and Technology Gaithersburg,

MD, September

Grill, R A., and D A Johnson 2005 Federal Building and Fire Safety Investigation of the World

Trade Center Disaster: Fire Protection and Life Safety Provisions Applied to the Design and

Construction of World Trade Center 1, 2, and 7 and Post-Construction Provisions Applied after

Occupancy NIST NCSTAR 1-1D National Institute of Standards and Technology Gaithersburg,

MD, September

Razza, J C., and R A Grill 2005 Federal Building and Fire Safety Investigation of the World

Trade Center Disaster: Comparison of Codes, Standards, and Practices in Use at the Time of the

Design and Construction of World Trade Center 1, 2, and 7 NIST NCSTAR 1-1E National

Institute of Standards and Technology Gaithersburg, MD, September

Grill, R A., D A Johnson, and D A Fanella 2005 Federal Building and Fire Safety

Investigation of the World Trade Center Disaster: Comparison of the 1968 and Current (2003) New

Preface

York City Building Code Provisions NIST NCSTAR 1-1F National Institute of Standards and

Technology Gaithersburg, MD, September

Grill, R A., and D A Johnson 2005 Federal Building and Fire Safety Investigation of the World Trade Center Disaster: Amendments to the Fire Protection and Life Safety Provisions of the New York City Building Code by Local Laws Adopted While World Trade Center 1, 2, and 7 Were in Use NIST NCSTAR 1-1G National Institute of Standards and Technology Gaithersburg, MD,

September

Grill, R A., and D A Johnson 2005 Federal Building and Fire Safety Investigation of the World Trade Center Disaster: Post-Construction Modifications to Fire Protection and Life Safety Systems

of World Trade Center 1 and 2 NIST NCSTAR 1-1H National Institute of Standards and

Technology Gaithersburg, MD, September

Grill, R A., D A Johnson, and D A Fanella 2005 Federal Building and Fire Safety Investigation

of the World Trade Center Disaster: Post-Construction Modifications to Fire Protection, Life Safety, and Structural Systems of World Trade Center 7 NIST NCSTAR 1-1I National Institute of

Standards and Technology Gaithersburg, MD, September

Grill, R A., and D A Johnson 2005 Federal Building and Fire Safety Investigation of the World Trade Center Disaster: Design, Installation, and Operation of Fuel System for Emergency Power in World Trade Center 7 NIST NCSTAR 1-1J National Institute of Standards and Technology

Gaithersburg, MD, September

Sadek, F 2005 Federal Building and Fire Safety Investigation of the World Trade Center Disaster: Baseline Structural Performance and Aircraft Impact Damage Analysis of the World Trade Center Towers NIST NCSTAR 1-2 National Institute of Standards and Technology Gaithersburg, MD,

September

Faschan, W J., and R B Garlock 2005 Federal Building and Fire Safety Investigation of the World Trade Center Disaster: Reference Structural Models and Baseline Performance Analysis of the World Trade Center Towers NIST NCSTAR 1-2A National Institute of Standards and

Technology Gaithersburg, MD, September

Kirkpatrick, S W., R T Bocchieri, F Sadek, R A MacNeill, S Holmes, B D Peterson,

R W Cilke, C Navarro 2005 Federal Building and Fire Safety Investigation of the World Trade

Center Disaster: Analysis of Aircraft Impacts into the World Trade Center Towers, NIST

NCSTAR 1-2B National Institute of Standards and Technology Gaithersburg, MD, September

Gayle, F W., R J Fields, W E Luecke, S W Banovic, T Foecke, C N McCowan, T A Siewert, and

J D McColskey 2005 Federal Building and Fire Safety Investigation of the World Trade Center Disaster: Mechanical and Metallurgical Analysis of Structural Steel NIST NCSTAR 1-3 National

Institute of Standards and Technology Gaithersburg, MD, September

Luecke, W E., T A Siewert, and F W Gayle 2005 Federal Building and Fire Safety

Investigation of the World Trade Center Disaster: Contemporaneous Structural Steel

Specifications NIST Special Publication 1-3A National Institute of Standards and Technology

Gaithersburg, MD, September

Preface

Banovic, S W 2005 Federal Building and Fire Safety Investigation of the World Trade Center

Disaster: Steel Inventory and Identification NIST NCSTAR 1-3B National Institute of Standards

and Technology Gaithersburg, MD, September

Banovic, S W., and T Foecke 2005 Federal Building and Fire Safety Investigation of the World

Trade Center Disaster: Damage and Failure Modes of Structural Steel Components NIST

NCSTAR 1-3C National Institute of Standards and Technology Gaithersburg, MD, September

Luecke, W E., J D McColskey, C N McCowan, S W Banovic, R J Fields, T Foecke,

T A Siewert, and F W Gayle 2005 Federal Building and Fire Safety Investigation of the World

Trade Center Disaster: Mechanical Properties of Structural Steels NIST NCSTAR 1-3D

National Institute of Standards and Technology Gaithersburg, MD, September

Banovic, S W., C N McCowan, and W E Luecke 2005 Federal Building and Fire Safety

Investigation of the World Trade Center Disaster: Physical Properties of Structural Steels NIST

NCSTAR 1-3E National Institute of Standards and Technology Gaithersburg, MD, September

Evans, D D., R D Peacock, E D Kuligowski, W S Dols, and W L Grosshandler 2005 Federal

Building and Fire Safety Investigation of the World Trade Center Disaster: Active Fire Protection

Systems NIST NCSTAR 1-4 National Institute of Standards and Technology Gaithersburg, MD,

September

Kuligowski, E D., D D Evans, and R D Peacock 2005 Federal Building and Fire Safety

Investigation of the World Trade Center Disaster: Post-Construction Fires Prior to September 11,

2001 NIST NCSTAR 1-4A National Institute of Standards and Technology Gaithersburg, MD,

September

Hopkins, M., J Schoenrock, and E Budnick 2005 Federal Building and Fire Safety Investigation

of the World Trade Center Disaster: Fire Suppression Systems NIST NCSTAR 1-4B National

Institute of Standards and Technology Gaithersburg, MD, September

Keough, R J., and R A Grill 2005 Federal Building and Fire Safety Investigation of the World

Trade Center Disaster: Fire Alarm Systems NIST NCSTAR 1-4C National Institute of Standards

and Technology Gaithersburg, MD, September

Ferreira, M J., and S M Strege 2005 Federal Building and Fire Safety Investigation of the

World Trade Center Disaster: Smoke Management Systems NIST NCSTAR 1-4D National

Institute of Standards and Technology Gaithersburg, MD, September

Gann, R G., A Hamins, K B McGrattan, G W Mulholland, H E Nelson, T J Ohlemiller,

W M Pitts, and K R Prasad 2005 Federal Building and Fire Safety Investigation of the World Trade

Center Disaster: Reconstruction of the Fires in the World Trade Center Towers NIST NCSTAR 1-5

National Institute of Standards and Technology Gaithersburg, MD, September

Pitts, W M., K M Butler, and V Junker 2005 Federal Building and Fire Safety Investigation of

the World Trade Center Disaster: Visual Evidence, Damage Estimates, and Timeline Analysis

NIST NCSTAR 1-5A National Institute of Standards and Technology Gaithersburg, MD,

September

Preface

Hamins, A., A Maranghides, K B McGrattan, E Johnsson, T J Ohlemiller, M Donnelly,

J Yang, G Mulholland, K R Prasad, S Kukuck, R Anleitner and T McAllister 2005 Federal Building and Fire Safety Investigation of the World Trade Center Disaster: Experiments and

Modeling of Structural Steel Elements Exposed to Fire NIST NCSTAR 1-5B National Institute of

Standards and Technology Gaithersburg, MD, September

Ohlemiller, T J., G W Mulholland, A Maranghides, J J Filliben, and R G Gann 2005 Federal Building and Fire Safety Investigation of the World Trade Center Disaster: Fire Tests of Single Office Workstations NIST NCSTAR 1-5C National Institute of Standards and Technology

Gaithersburg, MD, September

Gann, R G., M A Riley, J M Repp, A S Whittaker, A M Reinhorn, and P A Hough 2005

Federal Building and Fire Safety Investigation of the World Trade Center Disaster: Reaction of Ceiling Tile Systems to Shocks NIST NCSTAR 1-5D National Institute of Standards and

Technology Gaithersburg, MD, September

Hamins, A., A Maranghides, K B McGrattan, T J Ohlemiller, and R Anleitner 2005 Federal Building and Fire Safety Investigation of the World Trade Center Disaster: Experiments and

Modeling of Multiple Workstations Burning in a Compartment NIST NCSTAR 1-5E National

Institute of Standards and Technology Gaithersburg, MD, September

McGrattan, K B., C Bouldin, and G Forney 2005 Federal Building and Fire Safety

Investigation of the World Trade Center Disaster: Computer Simulation of the Fires in the World Trade Center Towers NIST NCSTAR 1-5F National Institute of Standards and Technology

Gaithersburg, MD, September

Prasad, K R., and H R Baum 2005 Federal Building and Fire Safety Investigation of the World Trade Center Disaster: Fire Structure Interface and Thermal Response of the World Trade Center Towers NIST NCSTAR 1-5G National Institute of Standards and Technology Gaithersburg,

MD, September

Gross, J L., and T McAllister 2005 Federal Building and Fire Safety Investigation of the World Trade Center Disaster: Structural Fire Response and Probable Collapse Sequence of the World Trade Center Towers NIST NCSTAR 1-6 National Institute of Standards and Technology Gaithersburg, MD,

September

Carino, N J., M A Starnes, J L Gross, J C Yang, S Kukuck, K R Prasad, and R W Bukowski

2005 Federal Building and Fire Safety Investigation of the World Trade Center Disaster: Passive Fire Protection NIST NCSTAR 1-6A National Institute of Standards and Technology

Gaithersburg, MD, September

Gross, J., F Hervey, M Izydorek, J Mammoser, and J Treadway 2005 Federal Building and Fire Safety Investigation of the World Trade Center Disaster: Fire Resistance Tests of Floor Truss Systems NIST NCSTAR 1-6B National Institute of Standards and Technology Gaithersburg,

MD, September

Zarghamee, M S., S Bolourchi, D W Eggers, Ö O Erbay, F W Kan, Y Kitane, A A Liepins,

M Mudlock, W I Naguib, R P Ojdrovic, A T Sarawit, P R Barrett, J L Gross, and

Preface

T P McAllister 2005 Federal Building and Fire Safety Investigation of the World Trade Center

Disaster: Component, Connection, and Subsystem Structural Analysis NIST NCSTAR 1-6C

National Institute of Standards and Technology Gaithersburg, MD, September

Zarghamee, M S., Y Kitane, Ö O Erbay, T P McAllister, and J L Gross 2005 Federal

Building and Fire Safety Investigation of the World Trade Center Disaster: Global Structural

Analysis of the Response of the World Trade Center Towers to Impact Damage and Fire NIST

NCSTAR 1-6D National Institute of Standards and Technology Gaithersburg, MD, September

McAllister, T., R W Bukowski, R G Gann, J L Gross, K B McGrattan, H E Nelson, L Phan,

W M Pitts, K R Prasad, F Sadek 2006 Federal Building and Fire Safety Investigation of the World

Trade Center Disaster: Structural Fire Response and Probable Collapse Sequence of World Trade

Center 7 (Provisional) NIST NCSTAR 1-6E National Institute of Standards and Technology

Gaithersburg, MD

Gilsanz, R., V Arbitrio, C Anders, D Chlebus, K Ezzeldin, W Guo, P Moloney, A Montalva,

J Oh, K Rubenacker 2006 Federal Building and Fire Safety Investigation of the World Trade

Center Disaster: Structural Analysis of the Response of World Trade Center 7 to Debris Damage

and Fire (Provisional) NIST NCSTAR 1-6F National Institute of Standards and Technology

Gaithersburg, MD

Kim, W 2006 Federal Building and Fire Safety Investigation of the World Trade Center

Disaster: Analysis of September 11, 2001, Seismogram Data (Provisional) NIST NCSTAR 1-6G

National Institute of Standards and Technology Gaithersburg, MD

Nelson, K 2006 Federal Building and Fire Safety Investigation of the World Trade Center

Disaster: The Con Ed Substation in World Trade Center 7 (Provisional) NIST NCSTAR 1-6H

National Institute of Standards and Technology Gaithersburg, MD

Averill, J D., D S Mileti, R D Peacock, E D Kuligowski, N Groner, G Proulx, P A Reneke, and

H E Nelson 2005 Federal Building and Fire Safety Investigation of the World Trade Center Disaster:

Occupant Behavior, Egress, and Emergency Communication NIST NCSTAR 1-7 National Institute of

Standards and Technology Gaithersburg, MD, September

Fahy, R., and G Proulx 2005 Federal Building and Fire Safety Investigation of the World Trade

Center Disaster: Analysis of Published Accounts of the World Trade Center Evacuation NIST

NCSTAR 1-7A National Institute of Standards and Technology Gaithersburg, MD, September

Zmud, J 2005 Federal Building and Fire Safety Investigation of the World Trade Center

Disaster: Technical Documentation for Survey Administration NIST NCSTAR 1-7B National

Institute of Standards and Technology Gaithersburg, MD, September

Lawson, J R., and R L Vettori 2005 Federal Building and Fire Safety Investigation of the World

Trade Center Disaster: The Emergency Response Operations NIST NCSTAR 1-8 National Institute of

Standards and Technology Gaithersburg, MD, September

contributions

Volunteers of the Structural Engineers Association of New York are thanked for their efforts in the steel recovery: Amit Bandyopadhyay, Anamaria Bonilla, Peter Chipchase, Anthony Chuliver, Edward

DePaola, Louis Errichiello, James Fahey, Jeffrey Hartman, David Hoy, Dean Koutsoubis, Andew

McConnell, Rajani Nair, Alan Rosa, Gary Steficek, Kevin Terry Countless hours were unselfishly spent searching for these invaluable pieces that are an integral component of this investigation Further, the following people are recognized for outstanding leadership roles in the recovery effort:

• Ramon Gilsanz, primary leader of the recovery effort

• David Sharp, coordinated volunteer activities; authored guide selection of suitable pieces

• Audrey Massa of the Federal Emergency Management Agency (FEMA), documentation and cataloguing of efforts and pieces to be saved

The FEMA/American Society of Civil Engineers Building Performance Study team, Professor A

Astaneh-Asl (University of California, Berkeley), and the National Science Foundation are recognized for their help in the recovery effort; Blanford Land Development Corporation, Hugo Neu Schnitzer, Inc., and Metal Management, Inc., for their assistance and patience during the review, abatement, and removal of pieces of interest from the salvage yards; and the Port Authority of New York and New Jersey for its assistance in surveying, sectioning, and transporting the technically significant steel collected and stored

at John F Kennedy International Airport

Dr George Krauss (University Emeritus Professor, Colorado School of Mines), Dr Elliot Brown (EB Consulting, Golden, CO), and Mr Arlan O Benscoter and Dr Arnold R Marder (Lehigh University), are recognized for technical discussions regarding the metallurgy of the steels in the World Trade Center (WTC) towers

Professor Hugh MacGillivray (Imperial College, London, UK) and Professor David Matlock (Colorado School of Mines) are recognized for technical discussions regarding high strain rate testing, and Professor Matlock for use of the high-rate tensile testing machine at the Colorado School of Mines

Dr Amelia Logan and Dr Edward P Vicenzi of the National Museum of Natural History, Smithsonian Institution, are thanked for analytical examination of the corrosion products observed on the recovered WTC steel

Acknowledgments

Dr Kazushige Tokuno (Nippon Steel USA) is recognized for supplying historical information on Yawata

steel

Former employees of Laclede Steel (St Louis, Missouri) are recognized, including David McGee and

Larry Hutchison, for locating Laclede records for NIST investigators during a visit to the Laclede steel

mill

Professors John Fisher and Al Pense (Lehigh University), and Drs Keith Taylor and Bruce Bramfitt

(International Steel Group), are thanked for discussions of WTC era steels and properties

E XECUTIVE S UMMARY

E.1 OVERVIEW

The World Trade Center (WTC) towers collapsed on September 11, 2001, as a result of damage inflicted

by aircraft and the ensuing fires The properties of the steel played an important role in how the building performed, from the initial impact to the final collapse Structural steel recovered from the site has been a valuable resource in the investigation of the disaster, providing information ranging from details of structural response to the aircraft impact to data on steel properties for insertion into models of building performance

This report is an overview of the mechanical and metallurgical analysis of structural steel from the WTC, part of the National Institute of Standards and Technology (NIST) Investigation of the WTC disaster The purpose was to analyze structural steel available from WTC 1, 2, and 7 to determine the metallurgical and mechanical properties and quality of the metal, weldments, and connections, and provide these data to other parts of the NIST WTC Investigation for insertion into models of building performance The analysis focused on steel from the towers to provide data for modeling building performance and

characterizing the steel quality In addition, the steel used in the construction of WTC 7 is described based solely on data from the literature, because no steel from the building was recovered

The three goals were to:

1 Determine mechanical properties of WTC structural steel,

2 Determine the quality of the steel and if it met its design requirements, and

3 Analyze and provide insight into failure mechanisms for guiding the development of models

of building performance and validating their output

E.1.1 Building Design and Steel Specifications

Building plans and material specifications from the construction era provided a starting point for the study Thousands of pages of design documents were reviewed Most valuable were the structural steel design drawings for the WTC towers provided by the Port Authority of New York and New Jersey In addition, Laclede Steel Company, the fabricator of the floor trusses, provided construction-era documents that showed, amongst other information, that steels with higher strength than specified were used in the floor truss systems Numerous other sources, including Yawata (now Nippon) Steel documents on

perimeter column steel, provided essential insights into the steel fabrication and properties

From the standpoint of building design, the towers were unique based on the number of different steels specified for construction Fourteen different strengths of steel were specified in the design drawings, although only 12 were actually used Most modern buildings use no more than two or three different strengths of steel

Executive Summary

Furthermore, more than a dozen suppliers and fabricators supplied steel for the buildings As a result, even when the different steels met a single specification, their properties could be significantly different These complications resulted in more than forty different steels being used in the tower structures, all of which are characterized to some extent in this study

E.2 INVENTORY OF RECOVERED STEEL

A total of 236 recovered pieces of WTC steel were cataloged; the great majority belonging to the towers, WTC 1 and WTC 2 These samples represented a quarter to half a percent of the 200,000 tons of

structural steel used in the construction of the two towers The NIST inventory included pieces from the impact and fire regions, perimeter columns, core columns, floor trusses, and other pieces such as truss seats and wind dampers

The original, as-built locations of 42 recovered perimeter panels and 12 recovered core columns were determined, based on markings and geometry of the columns Samples were available of all 12 strength levels of perimeter panel steel, the two strength levels of the core column steel that represented 99 percent

of the total number of columns, and both strength levels used in the floor trusses

A number of structural pieces were recovered from locations in or near the impact- and fire-damaged regions of the towers, including four perimeter panels directly hit by the airplane and three core columns located within these areas These pieces provided opportunity for failure and other forensic analyses

The collection of steel from the WTC towers was sufficient for determining the quality of the steel and, in combination with published literature, for determining mechanical properties as input to models of

building performance

E.3 DAMAGE AND FAILURE MODES OF THE STRUCTURAL STEEL

Extensive failure analysis of the recovered steel was conducted to determine damage characteristics, failure modes, and fire-related degradation of the recovered structural components In addition, pre-collapse photographic evidence of the impact damage and location and intensity of the fires was used to distinguish between pre- and post-collapse damage

Two sets of observations were made:

1 Pre-collapse analysis concentrated on impact damage sustained by the perimeter panel

sections The analysis employed enhanced photographic and video images of the towers and was largely limited to the perimeter panels, because the core columns and most floor trusses are not visible in the images The images were also used to determine the location of pre-collapse fires, damage to fire proofing, and possible fire-related damage to the panels In addition, some details of pre-collapse, time-dependent deformation of the structure, including bowing of perimeter columns across wide areas of the perimeter walls, were characterized

2 Post-collapse analysis concentrated on the damage characteristics of the recovered structural steel elements The perimeter panels were again evaluated, as well as the core columns and the connectors or seat assemblies used to attach the floor trusses to the core columns and